The invention relates to the machining and drilling of holes in composite materials mainly of the class characterized by being filament or fiber reinforced plastics. These composite materials are finding wide application in aircraft industries and in space programs of the National Aeronautics and Space Administraction. However, continued use of composite materials requires routine experimental investigation to determine specimen properties such as mechanical, strength, elastic, etc. Most important of the properties is the in-plane shear strength which is a property of composite materials such as graphite/epoxy which needs to be routinely determined.
Shear tests typically require drilling of holes in the composite materials. Due to the nature of composite materials, the problem occurs routinely that damage to the drill bits as well as to the materials occur during the boring of holes in the materials which are required for performing the above tests. Typically, a conventional drill press is utilized to drill these bores in the material. Even when silicon carbide or low-carbon steel drill bits are utilized, the damage to the drill bits still occurs. No conventional drill bits have been found on the market which can stand the rigors of routine drilling of the holes in the composite materials heretofore described without damaging the bit. In testing the composite materials for the above noted properties, a test commonly referred to as the "three-rail shear test" is commonly employed. This test is discussed in Composite Material Design Guide, University of Delaware, Vol. 1, Newark, Del. (1983), and in Experimental Mechanics of Fiber Reinforced Composite Materials, by J. M. Whitney, et al., Prentiss-Hall, Brookfield, Conn., Pg, 160 (1982). In performing the three-rail shear test, nine holes of 1/2 inch diameter are drilled through the thickness of a broad of composite material. The thickness of the board depends on the nature of the particular composite material. For the above referred to graphite/epoxy composite material, a board of 0.06 inches is utilized. During preparation of the sample board, considerable trouble may be encountered in attempting to bore the nine holes with conventional drill bits such as described above. Various combinations of tool speed and pressures may be used, however, even when using water cooling, the end result is usually an unusable hole.
Accordingly, an object of the present invention is to provide a method for drilling test holes in a board of composite material during a three-rail shear test without damaging the drill tool or the material specimen.
Another object of the present invention is to provide a method for machining a hole in a composite material of the fiber-reinforced plastic type without damaging the drill tool or the material.
Still another object of the invention is to provide a method for drilling or machining a uniform regular bore in a test specimen of composite fiber-reinforced plastic material so that accurate testing of the material for shear strength may be had.